Design of Molybdenum Oxide Charge Transport Layer for Crystalline Silicon Solar Cell

Abstract

The Sun is a huge source of renewable energy for mankind yet its benefits have not been effectively harnessed into primary mode of electricity generation. Although photovoltaic (PV) technology has been the major source of power generation in some of the developed economies, nevertheless there is a long way to go in achieving a significant grid parity worldwide especially in developing countries. Photovoltaic (PV) researchers and industries are actively striving to design and develop silicon (Si) solar cells that can offer at par conversion efficiency (η) with improved reliability. The energy conversion efficiency (η) of 26.6% attained for Si PV is based on the heterojunction technology. Although this efficiency is within the reach of Quessier Shockly limit of 29.4% for Si solar cells, complications involved in the industrial production along with complex lithographic patterning techniques are among the key barriers preventing its widespread implementation. Moreover, utilization of p-type amorphous silicon (a-Si:H) as a doped film for forming solar cell junction is prone to performance deterioration within the solar cell. The presence of defects states in a-Si:H degrade carrier mobility as well as its higher absorption coefficient leading towards unnecessary parasitic absorption of sunlight that reduces the solar cell current output. Forming a doping region with traditional techniques is a temperature-intensive process that escalates the thermal cost associated with the solar cell’s fabrication process. A device based on transition metal oxide of molybdenum oxide (MoOx, x<3) has been proposed instead of doped film whereby dopant-free MoOx film has been grown using reactive radio frequency (RF) sputtering technique. MoOx offers enhanced charge transport properties, larger work function (ψ) as well as energy band gap (Eg). With larger band gap, more sunlight can reach the underlying absorber, thus mitigating the absorption losses whereas larger work function technically transforms to lowering of the hole extraction barrier at the anode interface. The barrier reduction allows holes to be transported towards anode terminal via tunneling. Hence, MoOx is also known as a hole-selective contact. Since there are no dopant atoms involved in the hole transport layer of MoOx, therefore, all doping-related limitation like the heating issues, carrier scattering, Auger recombination and dopant precipitations are predominantly non-existent. In contrast to the thermal diffusion or chemical vapor deposition processes, no precursors are required for the growth of dopant-free regions. ii Following the literature review of Si solar cell technology and MoOx hole transport layer, Si heterostructure solar cell featuring MoOx film has been physically modelled for the first time in the literature using Silvaco TCAD simulator. The simulation provided physical insights into the operational mechanism of the device as a result of evaluating several device parameters. With the larger work function, electrons have to face Schottky barrier at the interface thereby reducing the recombination. Increasing the MoOx thickness significantly altered the band configuration that resulted in the tunnelling of minority carriers leading to more recombination. By optimisation of the parameters, the solar cell demonstrated higher open-circuit voltage (Voc) of 752 mV, short-circuit current density (Jsc) of 38.8 mA/cm2, fill factor (FF) of 79.0%, and η of 25.6%. Along with the simulations, experiments have also been undertaken in order to get indepth understanding about optical and electrical properties of MoOx as grown by reactive RF sputtering technique. The films were fabricated with various oxygen flow rates and characterized by stoichiometry, work function, interfacial defects states, optical dispersion data, transmittance, and band gap. X-ray Photoelectron Spectroscopy (XPS) studies have shown that as-deposited MoOx contained Mo5+ and Mo6+ oxidation states. At low oxygen flow rate, lower stoichiometry and work function values were observed that increased with an increasing oxygen flow rate. Consequently, highest work function of 5.92 eV was achieved for RF-sputtered MoOx films. The stoichiometry for MoOx was found to be 2.73. The capacitancevoltage (CV) analysis have unveiled the reduction in defects states of MoOx with increasing oxygen content. Refractive index for as-deposited films ranged from ~1.8-2.05. Similarly, transmittance and band gaps were found to increase correspondingly with an increase in oxygen flow rate. Further analysis on annealing and lifetime studies established an optimal annealing temperature of 1700C. The preliminary solar cell device based on back-junction configuration with RF-sputtered and thermal-evaporated MoOx were also fabricated and η of 2.1% and 10.9% was achieved, respectively, and further enhancement is conceivable by optimization techniques. The thesis work will prove to be instrumental in designing and fabrication of an efficient dopant-free solar cell device for industrial production as well as paving way for further research into other dopant-free solar cells.